It is proposed that the National Institutes of Health fund for a period of 12 months a theoretical and experimental medical physics program designed to determine the extent to which a strong transverse external magnetic field can be employed to enhance high energy electron LET in tumors. Preliminary calculations strongly indicate that it is possible with existing superconductor technology to localize a beam of high energy electrons to a region of approximately 2 cc. The objectives of the theoretical feasibility study are: (1) to determine the optimum design parameter (such as the minimum fringe-field, shielding, and cost) for the superconducting magnet-system necessary for the construction of a full-size clinical model; (2) upon completion of objective 1, to calculate the enhancement of the electron absorbed depth dose curve due to the presence of the magnetic field in the tumor region. These calculations would take into account electron energy straggling and lateral diffusion. Due to the expense of the optimum superconducting magnet-system necessary for clinical use, it is proposed that a scaled-down experimental study be undertaken. The proposed experimental dosimetry measurements would be made using the Washington University Medical School 35 MeV Varian Accelerator and a tissue-equivalent polystyrene phantom (with film), and a magnet capable of producing a 20-30 KG field over a volume of approximately 10 cc. The objectives of these measurements are (1) to demonstrate experimentally that the magnetic field does, as expected, concentrate the electron absorbed dose in a selected volume of the phantom; (2) to obtain a better understanding of the physics involved and compare the measured enhancements with the calculated ones obtained from the theoretical study; (3) if warranted, to have available sound experimental evidence in favor of the future support of a program to develop and test a full-size clinical superconducting magnet-system.